Here's an area where I feel like an expert.... sinking! :)
I think everyone has brought up very good points.
I think that the stronger your thigh and calve muscles are the more prone you are to having this problem due to the weight/size/density of the leg muscles.
Being a ex-baseball/sprinter I have more thigh mass than my fellow lifelong swimmers would. My legs definitely create drag and sink. The key is too develop your back muscles with the TI drills.
By practicing the drills I am imprinting the coordination/technique while developing the muscles in the back. Specifically the long muscles in the erector spinae group. The stronger they are the better leverage you will use to pull your legs up while you are swimming. As you get tired, back muscles fatigue and drop your legs.
Try floating on your back, do your legs sink? I've noticed that it's really easy to keep myself afloat. My pelvis can't bend backwards due its natural action. In other words, I can fold myself forward and touch my toes but I can bend backward like a flexible Gymnast and reach the floor.
CharlesCouturier posted this video a few weeks ago and it clicked for me where I needed to practice http://www.youtube.com/watch?v=oW5nE5FBPsQ
Anyways, as with all things worth doing, this doesn't come easy but with lots of practice and patience. With dedication and a positive "can do" perspective you will get there at your own pace!
Swim, Smile, and Thrive :)